1. Field of the Invention.
The invention relates generally to electrical switches and more particularly to disconnect switches for use in isolating an electrical source from a three phase motor and reversing switches for reversing operation of a three phase motor.
Disconnect switches of the type contemplated by the patent are used to electrically isolate equipment such as generators, circuit breakers and step up transformers from their power source and reversing switches are used to operate a motor in either forward or reverse direction. When operating equipment in explosive environments, such as a coal mine, there is a need to ensure that the disconnect/reversing switch is explosion-proof to prevent electrical sparks from igniting methane gasses in the mine shaft. Therefore, any disconnect/reversing switch to be used in conjunction with coal mining equipment must meet explosion-proof enclosure requirements established by the Mine Safety and Health Administration (MSHA). Further, a disconnect/reversing switch attached to electrical equipment is required by national standards to provide means for connecting the load-side conductors of the motor to a ground source when the motor is disconnected from the electrical power source.
Explosion-proof operation is achieved by journaling the operating shaft of the disconnect/reversing switch in such a way as to provide sufficient axial length and minimum clearance between the shaft and operating bore such that any explosion within the switch resulting from sparks and a mixture of explosive gas, such as methane gas will be quenched by the time it exits the switch through the clearance between the shaft and bore.
2. Related Art.
A typical disconnect switch is described in U.S. Pat. No. 4,737,603 issued Apr. 12, 1988 to Jennings Lycan. The Lycan disconnect switch comprises a cam-operated handle assembly that moves the switch from open, load-grounded, to closed position. The disconnect switch contains two fixed members and a moveable member, wherein the moveable member has six contacts to engage sockets on either fixed member according to the position of the switch. Further, the motor is grounded by means of wire jumpers which connect the two fixed members. While this switch functions to disconnect a motor from its source of power, the cam-driven handle assembly is difficult to operate and the wire jumpers used to ground the motor are not an integral part of the unit. Further, an aperture insulator drum is required to secure the two fixed contact members an equal distance from each other, adding to the assembly costs.
A typical reversing switch is described in U.S. Pat. No. 4,563,549 issued Jan. 7, 1986 to Jennings Lycan. The Lycan reversing Switch comprises a cam-operated handle assembly that permits axial and rotational movement. The reversing switch comprises a fixed member containing four sockets and a moveable member having four contacts to engage the sockets on the fixed member. In operation, the moveable member must be rotated 90 degrees to reverse the operation of the motor. While the switch functions to reverse the operation of a motor, the cam-driver handle assembly is difficult to operate and requires a two-step operation to reverse the direction of the motor.
There is no disclosure of a combination disconnect/reversing switch in the prior art. This type of switch would be beneficial in situations where the moto is located a distance from the power source and where space is limited such that both switches may not be physically located at the same place at the same time. Further, the use of two switches is inefficient and costly when a single switch can accomplish both functions.